Field of the Invention
The present invention relates to an electrode material, an electrode, and a lithium ion battery, and particularly to an electrode material that is preferably used for positive electrode materials of lithium ion batteries for which a phosphate-based electrode active material having an olivine structure is used as an electrode active material and which is excellent in terms of charge characteristics, cycle characteristics and energy density, an electrode containing the above-described electrode material, and a lithium ion battery including a positive electrode made of the above-described electrode.
Description of Related Art
In recent years, non-aqueous electrolytic solution-based secondary batteries, such as lithium ion batteries, have been proposed and put into practical use as batteries having a small size, a light weight and a high capacity. The lithium ion battery is made up of a positive electrode, a negative electrode, both of which can reversibly insert and remove lithium ions, and a non-aqueous electrolyte.
Since lithium ion batteries have a light weight, a small size and a high energy compared with secondary batteries of the related art, such as lead batteries, nickel-cadmium batteries and nickel-hydrogen batteries, lithium ion batteries are used as a power supply for portable electronic devices such as mobile phones, notebook computers and portable information terminals; however, in recent years, have been studied as a high-output power supply for electric vehicles, hybrid vehicles, electric tools and the like. Electrode active materials used in batteries that are used as the high-output power supply are required to have high-speed charge and discharge characteristics.
In addition, studies are being made not only of the smoothing of the load of power generation but also of the applying of lithium ion batteries to large-sized batteries for a stationary power supply and a backup power supply, and the problemless resource amount as well as long-term stability and long-term reliability are emphasized.
Positive electrodes of lithium ion batteries are made of an electrode material containing a lithium-containing metal oxide that can reversibly insert and remove lithium ions, which is called a positive electrode active material, a conduction aid and a binder, and, when the electrode material is applied to the surface of a metallic foil called a collector, a positive electrode is produced. As the positive electrode active material for lithium ion batteries, lithium cobaltate (LiCoO2) is generally used; however, additionally, lithium (Li) compounds such as lithium nickelate (LiNiO2), lithium manganate (LiMn2O4) and lithium iron phosphate (LiFePO4) are used. Among the above-described compounds, lithium cobaltate and lithium nickelate have problems of instability in the charged state as well as lacking in the amount of available resources. In addition, for lithium manganate, a problem of being dissolved in an electrolytic solution at a high temperature is pointed out.
Therefore, in recent years, phosphate-based electrode materials having an olivine structure represented by lithium iron phosphate are drawing attention as electrode materials that are excellent in terms of long-term stability and long-term reliability.
However, since the phosphate-based electrode materials have insufficient electron conductivity, as a method of improving the electron conductivity of the electrode materials, a method has been proposed in which an oxidant is added to a solution containing aniline and a raw material of an active material in an aqueous solvent so as to generate fine particles of the active material, and the aniline is polymerized on the surfaces of the fine particles of the active material (Japanese Laid-open Patent Publication No. 2010-40357).
However, in the above-described method, there was a problem in that aniline had a low solubility in the aqueous solvent such that it was difficult for aniline to be selectively attached to the electrode material when aniline was polymerized in the solution, furthermore, there was a case in which the solubility changed along with polymerization, and consequently, the thickness of a polyaniline-coated film on the surface of the electrode material became uneven or particles made only of polyaniline were formed, aniline did not effectively contribute to the improvement of the electron conductivity of the material, and performance became irregular.
As described above, since the phosphate-based electrode active materials have insufficient electron conductivity, in order to charge and discharge large currents, a variety of attempts such as the miniaturization of the particles and the formation of a complex with a conductive material are required, and a number of efforts have thus far been made.
However, the formation of a complex through the minimization of the particles or the addition of a large amount of a conductive material causes a decrease in electrode density such that a new problem of a decrease in the density of batteries, that is, a decrease in the capacity per unit volume is caused. Therefore, as a method of solving the above-described problem, a carbon coating method is proposed in which a solution of an organic substance is used as a carbon precursor which is an electron conductive substance (for example, refer to Japanese Laid-open Patent Publication No. 2012-185979).
The above-described method is a method in which a solution containing particles of an electrode active material and an organic substance, which is a carbon source, is dried, and then thermally treated in a non-oxidizing atmosphere, thereby forming a thin layer containing an organic substance-derived carbonaceous electron conductive substance on the surfaces of particles of an electrode active material.
In the above-described method, it is possible to apply a necessary minimum amount of an electron conductive substance to the surfaces of the particles of an electrode active material with an extremely high efficiency, and therefore it is possible to improve the conductivity without causing a significant decrease in the density of electrodes.